This invention relates to a digital welder control system and more particularly to a digital welder control system for the automotive industry in which varying line voltages are automatically compensated for in the controller to maintain a constant RMS current in the welding transformer secondary.
Portable or stationary gun welders have been used for many years in the automotive industry, and recently the automation of the welding operation for assembly line work to provide adequate control of the welding sequence has been successfully accomplished by the digital welder control system of U.S. Pat. No. 4,104,724 (hereinafter '724 patent).
However, in assembly line work, typically a large number of these portable stationary resistance welders are operating randomly resulting in line voltage variations due to the fluctuating heavy loads imposed which interfere with the production of quality welds. The maintenance of constant current is an important factor in weld quality due to the square law relationship between the current and the heat delivered to the weld. Thus, small errors in the current have a large effect on the total heat energy put into the weld. Line voltage variations naturally have a direct affect on the weld current. If at some instant in time most of the welders are operating, the line voltage will sag excessively and as a result, the welds being produced will receive insufficient energy. Conversely, if only a few welders are operating, the line voltage will tend to be high, thus, causing the welds to be made with excess energy. Thus, a welder control system which senses the line voltage and adjusts itself to hold the welding RMS current constant is advantageous because it will tend to produce uniform weld quality in the presence of line voltage variations. Avoidance of overwelding and the corresponding improvement in tip life and power consumption are desirable characteristics.